This disclosure relates generally to the field of marine seismic data acquisition. More specifically, the disclosure relates to methods for acquiring marine seismic data in which the positions of seismic energy source(s) used to generate the seismic data are randomly geodetically distributed in order to improve quality of the data obtained.
Seismic surveying is known in the art for determining structures of rock formations below the earth's surface. Seismic surveying generally includes deploying an array of seismic sensors at the surface of the earth in a selected pattern, and selectively actuating a seismic energy source positioned near the seismic sensors. The energy source may be an explosive, a vibrator, or in the case of seismic surveying performed in the ocean, one or more air guns or water guns.
Seismic energy which emanates from the source travels through the earth formations until it reaches an acoustic impedance boundary in the formations. Acoustic impedance boundaries typically occur where the composition and/or mechanical properties of the earth formation change. Such boundaries are typically referred to as “bed boundaries”. At an acoustic impedance boundary, some of the seismic energy is reflected back toward the earth's surface, where it may be detected by one or more of the seismic sensors deployed on or below the surface when onshore, and in the water when offshore. Other portions of the energy are refracted and continue propagating in a generally downward direction until another impedance boundary is reached. Seismic signal processing known in the art has as an objective the determination of the depths, geographic locations and physical properties of rocks forming a bed boundary below the earth's surface. The depth and location of the bed boundaries is inferred from the travel time of the seismic energy to the acoustic impedance boundaries and back to the sensors at the surface.
Seismic surveying (marine seismic surveying) is performed in bodies of water suck as lakes or the ocean to determine the structure of earth formations below the water bottom. Marine seismic surveying known in the art includes having a vessel tow one or more seismic energy sources, and the same or a different vessel tow one or more “streamers”, which are arrays of seismic sensors forming part of or otherwise affixed to a cable. Typically, a seismic vessel will tow a plurality of such streamers arranged to be separated by a selected fixed or variable lateral distance from each other, in a pattern selected to enable relatively complete determination of geologic structures in three dimensions.
The signals detected by the seismic sensors at the earth's surface include components of seismic energy reflected at the bed boundaries, as previously explained. In addition, both coherent noise (noise which has a determinable pattern, such as may be caused by a ship propeller) and incoherent (random) noise may be present. The presence of such noise in the signals received by the seismic sensors reduces the signal-to-noise ratio (“SNR”) of the seismic signals of interest. An objective of seismologists, therefore, is to seek methods of eliminating the effects of noise on the signals detected by the sensors without appreciably reducing the true seismic signal component of the detected signals. The resolution of the resultant seismic data is typically dependent on the spatial sampling of the signal and the noise.
Prior art methods which have been used to reduce the effects of noise and acquire a higher quality seismic representation of a particular subsurface structure include using multiple actuations of the seismic source (multiple “firings” or “shots”) to record a plurality of sensor measurements from substantially the same subsurface structure, and then summing or “stacking” such measurements to enhance signal strength while substantially reducing the effects of random or incoherent noise. In most such techniques known in the art, the multiple firings are performed such that the source is disposed at regularly spaced positions, and signal processing of the recorded signals follows accordingly.
The idea of random spatial sampling rather than regular spatial sampling of the subsurface has been proposed as a way that can lead to improved resolution of the subsequent data. These design principles and theoretical justification come from a relatively new field of mathematics known as “compressive sampling”. See, e.g., Candes, E., Romberg J., and Tao T., (2006) Stable signal recovery from incomplete and inaccurate measurements. Communications on Pure and Applied Mathematics 59, 1207-1223. See also, Donoho, D. L., (2006) Compressed sensing; IEEE Transactions on Information Theory. 52, 1289-1306.
Typically the approach of random sampling has been suggested as a way to obtain more information from fewer samples and has been considered in a theoretical sense for seismic data. See, Herrman, F., (2009) Sub-Nyquist sampling and sparsity: how to get more information from fewer samples, Proceedings of the 2009 Annual SEG meeting 3410-3415.
For land seismic acquisition, it is relatively easy to randomize the spatial positions of shots. Randomizing the positions of sensors is also possible with wireless systems, though more conventional wired systems would limit the potential. The concept of using random sampling in a land environment to get the same result through acquisition of less data has been described in, Milton A., Trickett, S., and Burroughs L., (2011) Reducing acquisition costs with random sampling and multi-dimensional interpolation, Proceedings of the 2011 Annual SEG meeting 52-56.
In the marine environment seismic data are typically acquired in straight lines with a set of sensors towed behind the vessel. There is in effect no capability to vary the relative positions of the sensors as these are constrained within a streamer towed behind the vessel. Some natural randomization of sensor positions may occur simply through the deviation of the streamers from the intended track due to currents, but the spacing of the sensor positions within the streamer is fixed.
Marine seismic sources are typically fired sequentially and alternately (in the case of 2 sources and a single vessel), with the objective of firing the sources at regularly spaced locations along a designated vessel track. There are known deviations from this practice known in the art involving the number of sources being activated, and the timing of the source activation.
A first technique known in the art is that multiple sources are fired sequentially with small deviations in timing between firings in each sequence. See, e.g., U.S. Pat. No. 6,906,981 B2 issued to Vaage, entitled, Method and system for acquiring marine seismic data using multiple sources. The method disclosed in the foregoing patent still has as an objective acquiring seismic data on a regular spatial sampling basis and recording the data into discrete records of fixed time duration. By introducing slight variations in the actuation timing of the secondary source some variation in the position of the second source is obtained, however such position randomization is relatively small. The purpose of the technique disclosed in the foregoing patent is to achieve randomization of source firing timing so that essentially simultaneously operated sources can have their energy individually identified and separated from the recorded seismic signals in a single discrete record. Note that for purposes of identifying the source position, the sources are actuated at essentially the same time, and at regularly spaced apart spatial positions.
Another technique known in the art provides that the track of the vessel not be straight, but be approximately circular. This essentially creates a pseudo random set of resultant source positions, but the seismic energy sources are still fired at regular spatial intervals along the vessel track. This is described in, Moldoveannu, N., (2010) Random Sampling: A new strategy for marine acquisition, Proceedings of the 2010 Annual SEG meeting 51-54. Note however, that using the foregoing technique the source positions are not randomized along the vessel track, but it is simply a result of the fact that the vessel track is not straight that results in spatial variation of the source position.
What is needed is a technique to randomize seismic energy source position for marine seismic data acquisition to obtain the benefits thereof.